Integrated Cold Start of a Boost Converter at 57 mV Using Cross-Coupled Complementary Charge Pumps and Ultra-Low-Voltage Ring Oscillator

This paper demonstrates an on-chip electrical cold-start technique to achieve low-voltage and fast start-up of a boost converter for autonomous thermal energy harvesting from human body heat. An improved charge transfer through high gate-boosted switches by means of cross-coupled complementary charge pumps enables voltage multiplication of the low input voltage during cold start. The start-up voltage multiplier operates with an on-chip clock generated by an ultra-low-voltage ring oscillator. The proposed cold-start scheme implemented in a general-purpose 0.18-μm CMOS process assists an inductive boost converter to start operation with a minimum input voltage of 57 mV in 135 ms while consuming only 90 nJ of energy from the harvesting source, without using additional sources of energy or additional off-chip components.

I. Introduction

Thermal energy from human body heat is a ubiquitous source of energy, and unlike solar power, it can be harnessed irrespective of illumination conditions. As such, body heat is an ideal energy source for self-powered wearable devices [1]. Thermal energy can be converted to electrical energy using thermoelectric generators (TEG), the solid-state devices that generate a voltage from an applied temperature gradient () using the Seebeck effect [2]. A wearable form factor requires small-area TEGs, which generate only tens of millivolts from the small value (~1 °C–2 °C) between the skin and the ambient environment. A dc–dc step-up converter is needed to boost this small voltage to power electronics. While the low-voltage dc–dc converters have been demonstrated, initial start-up of the power converter at low input voltage is challenging without using additional energy sources, such as batteries [3], mechanical vibrations [4], or RF sources [5].